Pathogenic Rickettsia species are etiologic agents of some of the most severe bacterial diseases known to mankind worldwide. Among them are Rocky Mountain spotted fever and epidemic typhus caused respectively by R. rickettsii and R. prowazekii. Rickettsiae are obligate intracellular parasites characterized by tropism for vascular endothelium of their mammalian hosts. Accruing evidence indicates an important role for reactive oxygen species (ROS) in endothelial dysfunction and rickettsial pathogenesis. As an upholder of physiological homeostasis, endothelium maintains vessel tone and vascular permeability and regulates inflammatory responses to a plethora of noxious stimuli. Our published and preliminary data further suggest that: 1. Infection of endothelial cells (EC) induces the expression of heme oxygenase (HO)-1, an enzyme responsible for the degradation of pro-oxidant heme into biologically active molecules (bilirubin, carbon monoxide, and ferritin) with diverse cytoprotective functions;2. Increased expression of cyclooxygenase (Cox)-2 during infection with spotted fever group (SFG) rickettsiae results in enhanced prostaglandin (PG) secretion;and 3. There are significant differences in the abilities of SF and typhus group (TG) organisms to trigger ROS generation and activate anti-oxidant defense mechanisms, likely due to distinct differences in their intracytoplasmic behavior. These observations have led to the hypothesis that regulation of HO and Cox enzyme systems and interplay between HO and Cox signaling mechanisms in the vasculature are critical determinants of host cell survival, onset/degree of inflammation, and changes in vascular permeability, all of which are critical determinants of pathologic manifestations of rickettsioses. The proposed studies are focused on elucidating heretofore unknown regulatory mechanisms controlling redox homeostasis, acute inflammation, and vascular permeability using infection of cultured human EC (in vitro) and disseminated endothelial infection of susceptible mice strains (in vivo) with SFG (R. rickettsii, R. conorii and R. australis) and TG (R. prowazekii, R. typhi) as independent model systems. Employing Rickettsia strains with varying degree of virulence, cell imaging, isozyme-specific inhibitors/activators or dominant- negative mutants, siRNA-based knockdown, tissue-targeted over-expression, and knockout mice, we will address the following thematically and mechanistically interrelated specific aims: Aim 1 will (i). define similarities and/or potential differences in signaling mechanisms and functional consequences of infection-induced HO-1 using Sheila Smith (highly virulent), HLP (comparatively less pathogenic), and Iowa (avirulent) strains of R rickettsii, and (ii). investigate the regulation of Cox-2 and its role in the control of vascular permeability via production of PGs and rickettsial replication. Aim 2 will characterize the differential effects of SFG versus TG rickettsiae and representative strains with varying degree of virulence within individual species on endothelial HO-1 and Cox-2 and identify specific rickettsial mechanisms that regulate host cell responses. Aim 3 will determine the role of HO-1 in the host-protective adaptations and Cox-2 in the pathophysiology of in vivo SFG and TG rickettsioses. Together, these studies will dissect the physiological significance and potential differences in the contributions of vasoactive products of HO-1 and Cox-2 activities in the pathogenetic and virulence mechanisms of SFG and TG rickettsioses and provide useful insight to identify novel therapeutic targets for species-specific interventions in particular or vasculopathic rickettsial diseases in general.